In astronomical satellites and the like, there has been conventionally employed a so-called sun-synchronous orbit in view of that the directions and relative positions of the sun and the earth viewed from the satellite can be kept constant. However, in the sun-synchronous orbits according to the conventional concept, since non-spherical components of the gravity, which are attributable to the oblateness of the earth, are used, conventional sun-synchronous orbits are realized only in a low-altitude orbit of approximately several hundred kilometers to a thousand kilometers, on which the satellites revolve around the earth (see the patent document 1), and the thermal radiation input from the earth cannot be avoided inevitably, thus this thermal radiation input from the earth is extremely disadvantageous to a cryogenically cooled telescope and the like in astronomical satellites. Moreover, when used as a communication satellite system, there is a drawback that the coverage thereof is narrower than that of a high-altitude geostationary satellite system, thus the sun-synchronous orbit based on the conventional concept is not used for a communication purpose. From the perspective of avoiding a thermal radiation input from the earth, it is necessary to extend the distance between the satellite and the earth, but according to the conventional concept of sun-synchronous orbits, the synchronous characteristics thereof apply only to the low-altitude orbit, thus this sun-synchronous condition cannot be satisfied by high-altitude orbits, i.e., long-period orbits.
Therefore, the distance between a satellite and the earth is sufficiently extended to Lagrangian points of the solar-terrestrial system to avoid the thermal radiation from the earth, whereby the geometry between the earth and the sun can be kept constant again. As a mission placing a satellite at the Lagrangian points L2, there is known the NASA's James Webb Space Telescope (JWST) satellite. JWST is inserted into a HALO orbit drifting in the vicinity of L2 point, but a problem in this orbit is in the fact that the geometric conditions in terms of the directions of the sun and the earth are not limited to one direction in principle. Also, there is a problem that the sensitivity to the radio observation information with respect to a position fluctuation of a satellite is small, it is required to accumulate the range measurements on the ground to determine the orbit solution and to secure the navigational accuracy.
From the communication distance point of view, the Lagrangian points which can be used are limited to L1 and L2 points, which are located relatively short distance away from the earth. But since these points are theoretically unstable equilibrium points, these points have to be constantly stabilized actively by performing an orbit determination and an orbit control. Particularly these points are easily affected by the moon's gravity. Further, a trajectory planning, which is more rigorous than that in a normal interplanetary probe, needs to be applied in the initial stage of the insertion, and thus it requires a long period to complete the orbit insertion. Moreover, it is difficult to secure astronomical observation time until the orbit insertion is completed. In the conventional concept, the effect of perturbation caused by non-spherical components of the gravity associated with the earth cannot be expected when the altitude of the sun-synchronous orbit becomes higher, while non-spherical components of the gravity usually generate the sun-synchronous effect, hence it is difficult to satisfy the sun-synchronous condition for the case. As one measure to solve this problem, it is conceived to place a satellite into an interplanetary orbit which flies in the vicinity of the earth. As an actually realized mission example, also as an interplanetary astronomical satellite, NASA's Space Infrared Telescope Facility (SIRTF, currently named Schpitzer) infrared observation satellite is known. In the SIRTF infrared observation satellite, there is selected, as an interplanetary orbit, whose orbital period is slightly altered different from that of the earth. However, in this orbit the distance between the earth and the satellite gradually increases year by year, thus there is a problem that it becomes gradually difficult to receive the data at higher bit rates. Moreover, while the angle between the sun direction and the earth direction, which is viewed from the satellite, the elongation angle of nearly up to 90 degrees is secured and available in the initial stage, such condition progressively becomes unsatisfactory, whereby it becomes gradually difficult to keep the direction of the sun, which is the light source and heat source, when the direction of the earth needs to be right.
Patent Document 1: Japanese Patent Application Laid-Open No. H10-258799